The invention concerns a transducer plate for piezo-electrical transducers in telephone technology with a carrier plate and a piezo-ceramic layer bound to it by means of adhesive applied on one side thereof. The piezo-ceramic layer is provided on both sides with electrodes.
Within the telephone transmission range of approximately 200 to 4,000 Hz, the frequency response of the telephone transducer must lie within a prescribed tolerance scheme. In order to satisfy this condition, it is known to place the fundamental resonance of the deflection vibration within the telephone transmission range, whereby the sensitivity of the transducer is raised to the necessary dimension. For broadening of the transmission range as well as for the boosting of the sensitivity, one further places the resonant frequency of the fourth partial oscillation, which is characterized by a node circle, at the upper boundary of the transmission range. Special resonators then have the task of attenuating the excessive rises in resonance to a given amount. Thus it is known to attenuate the fundamental resonance by means of absorption resonators arranged in the transducer. For the attenuation of the fourth partial oscillation, these absorption resonators however are not suitable since the small perforations required, for example in the transducer floor, are very difficult to produce, or can then easily become soiled, so that an operation with constant frequency response cannot be guaranteed. For this reason, the transducer plate was previously mounted in special bearing bodies (German Pat. No. 1,961,217 corresponding to U.S. Pat. No. 3,708,702, incorporated herein by reference). Further, one dimensioned the piezo-ceramic layer such that the node circle of the fourth partial oscillation develops within the piezo-layer.
As was mentioned above, the piezo-ceramic layer of the transducer plate is bound together with the transducer plate by means of gluing. The adhesive layer is designed as an extremely thin layer so that the adhesive hardens quickly and has no interfering influence on the frequency response of the transducer. It turned out that an adhesive layer of this sort brought with it problems with respect to stability as well as problems with respect to its use in automatic apparatus.